1. Field of the Invention
The invention relates generally to communications systems and more specifically to code position modulation multiple access schemes that use maximal length codes and exist to support several simultaneous users.
2. Description of the Prior Art
Radio communications system have evolved numerous ways to modulate information on a radio carrier. Amplitude, frequency, and phase modulation are some of the most common scheme. Both analog and digital techniques are used to communicate voice, video and computer data. The digital techniques are especially suited to computer data. Basically, communicating information over a radio carrier involves modulating the carrier. Some transmitted energy will be dispersed into sidebands by the modulation and it is these sidebands that carry the information. More power can be put into the sidebands if the carrier, which carries no information generally, is suppressed or eliminated. Similarly, only one of the two sidebands is strictly necessary, so single sideband (SSB) is an effective use of transmitter power.
Satellite communications are usually constrained by relatively low power levels because the generation of high power levels in an orbiting satellite is expensive and difficult to sustain. A class of communications called spread spectrum is particularly suited to satellite communications because it allows very small aperture (broad beam) antennas to be used on the ground without excessive interference to adjacent satellites other than the desired satellite. Spread spectrum communication uses the modulation to spread the radio carrier over a very wide range of frequencies.
Spread spectrum techniques have no special advantage or disadvantage with respect to operation under natural background noise conditions, except for the unique waveform of the present invention. A difficulty in all code division multiple access (CDMA) systems is control of the cross-user interference, which is the degrading or noise-like effects of undesired user signals on the signal to be received. To address a need for multiple user spread spectrum communication, code division multiple access (CDMA) evolved. However, cross-user noise remains a significant problem area in CDMA communications.
Horwitz describes in U.S. Pat. No. 4,644,523, issued Feb. 17, 1987, a way to improve the signal-to-noise ratio in a CDMA receiver using a direct sequence spread spectrum wherein a plurality of transmitters and at least one receiver are synchronized to a common timing source. Each transmitter sends a data signal spread by a bipolar pseudo-random code which is a different assigned shift of a common bipolar code sequence. The receiver has a plurality of correlation detectors, each generating two local bipolar pseudo-random codes that are replicas of the transmitter codes. One of the locally generated codes has the same code sequence shift as the code assigned to the predetermined transmitter. Horwitz notes that in spread spectrum communications spreading of the signal bandwidth beyond the bandwidth normally required for data being transmitted is done by phase shift keyed (PSK) modulating a carrier waveform by the data to be transmitted. Then the resultant signal is modulated by a reference pseudo-random code of length L running at a repetition rate which is normally twice the data rate. Demodulating involves heterodyning or multiplying the signal by the same reference code used to spread the composite transmission. Assuming that the transmitted and locally generated receiver codes are synchronous, the carrier inversions caused by the code PSK modulation are removed and the original base-band modulated carrier is restored by the receiver.
Another way to support multiple users is to separate them by assigning each a time slot period for communication. Such a scheme has been commonly used in satellite communications system. For example, Gilhousen, et al., were issued U.S. Pat. No. 4,979,170, on Dec. 18, 1990, which describes a message communication system having centralized communication stations that transmit messages through earth orbit repeater satellites to/from mobile terminals. The communication system uses time division multiple access (TDMA) with a number of channels designated address channels and the remaining channels used for data transfer. A primary problem of TDMA for small users is that since transmission is accomplished in a short, high-speed burst, the peak radio power must be high. In general, small, light-weight solid-state electronics are not able to generate transmit power this way.
CDMA systems can not support as many users as the number of code perturbations would seem to indicate as possible because of cross-user noise. Before all possible codes are being used, the cross-user noise becomes so great that all users' signals are degraded to the point of being unusable. Codes interact differently, depending on the particular codes involved. A great deal of study and experimentation has resulted in a limited set of codes and code combinations being preferred. The combinations that work best together are known to those skilled in the art as Gold Codes. The limited number of known Gold Codes constrains how many users can share the same channel, albeit with different codes. The signal-to-noise ratio (SNR) due to the cross-correlation of many codes against the desired receiver code does not exceed ##EQU1## where k is the number of simultaneous users and n is the length of the code. For example, with a code length of 511, the best possible SNR for ten simultaneous users is 3.5 dB. The number of Gold Codes that can actually be simultaneously used is insufficient for the number of simultaneous users in the field that are desirable to deploy, primarily because of cross-user interference.
A system and method of CDMA communication is needed that greatly expands the number of users possible in a single environment and that improves the signal separation between users and does not degrade the performance under natural background noise conditions.